This invention relates to a Rosen type piezoelectric transformer of the type proposed by R. A. Rosen.
In the manner which will later be exemplified with reference to the accompanying drawing, a conventional piezoelectric transformer comprises according to Rosen a thin piezoelectric element or plate which is made of a piezoelectric material, such as lead titanate zirconate (PTZ) and is typically rectangular in top view having a lengthwise direction, a widthwise direction, and a thickness direction orthogonal to each other. In general, a plurality of power electrodes are formed on the piezoelectric plate to provide a driver portion and a generator portion of the piezoelectric transformer with one and the other of the driver and the generator portions subjected to polarization in the lengthwise and the thickness directions.
In operation, a pair of the power electrodes on the driver portion are used as input electrodes. Another pair of the power electrodes are used as output electrodes usually with only one of the output electrodes placed on the generator portion and with another of the power electrodes used in common as one each of the input and the output electrodes. When driven by a transformer input signal supplied across the input electrodes, the piezoelectric transformer produces a transformer output signal across the output electrodes. Besides a few novel piezoelectric transformers, such conventional piezoelectric transformers are described in Japanese Patent Prepublication (A) No. 177,451 of 1994. The piezoelectric transformers are operable either in a first order Rosen type mode of vibration or in a third order Rosen type with an additional pair of input electrodes formed on the piezoelectric element.
More specifically, the transformer input signal excites a mechanical vibration in a direction of the polarization of the driver portion as a result of the piezoelectric effect when the transformer input signal has an input or drive frequency resonant with the mechanical vibration. The generator portion converts mechanical energy of the mechanical vibration to electric energy by its piezoelectric effect and produces the transformer output signal at the drive frequency.
The piezoelectric transformer is compact and has a thin thickness to be very convenient for use in energizing a cold cathode-ray tube to provide backlight of a liquid crystal display unit. The piezoelectric transformer may, however, produce the transformer output signal with an excessive output voltage beyond a predetermined output voltage, such as 2 kV, to damage the piezoelectric element. In order to avoid production of such an excessive output voltage, an improved driving circuit is disclosed in Japanese Patent Prepublication (A) No. 167,678 of 1996 for the conventional piezoelectric transformer.
Other than in the piezoelectric transformer, a piezoelectric element is used in various other applications. In Japanese Patent Prepublication (A) No. 4,769 of 1992, an ultrasonic motor is revealed. In the ultrasonic motor, a rotor is made of the piezoelectric material. A monitor electrode is attached to the rotor to detect an abnormal vibration of the rotor and thereby to control a driving signal supplied to the ultrasonic motor. For detection, the abnormal vibration must, however, be in a predetermined frequency range dependent on an electrode size.
In the manner described above, it has been known that damage, such as a crack, is caused to the piezoelectric transformer when the transformer output signal undesiredly has an excessive output voltage. The damage disturbs normal or homogeneous vibration of the piezoelectric element generating heat and reducing efficiency of the piezoelectric transformer. The inventors of the present invention have confirmed that such damage is caused also when the piezoelectric element is subjected to an abnormal vibration other than the normal vibration. The abnormal vibration has not been detected in the piezoelectric transformer.
It should be noted in connection with the foregoing that the conventional piezoelectric transformer has only one of the transformer output electrodes independently of the transformer input electrodes. Incidentally, the present inventors have confirmed that the abnormal vibration gives rise to an unbalance of vibration in the widthwise direction and that the unbalance occurs in the vibration in a direction parallel or orthogonal to polarization of the generator portion in the piezoelectric transformer operable in the vibration mode of the first order Rosen type and at an interface where the polarization of the generator portion is opposite in the piezoelectric transformer operable in the vibration mode of the third order Rosen type.